New Publications. 79 



a rav of white light, pafling between angular furfaces, fuch 

 as the almoft meeting edges of two knives, was liable to be 

 parted bv its bendings, feparatiOns, and other changes,' into 

 thofe differently coloured rays, which had been hippofed by- 

 Newton to be the primogenial elements of pure white iight. 

 This obfervation is here applied to explain the formation of 

 thofe irides or coi of ' mar, x- coloured circles, which 



are feen contiguous to the bodies of the fun, moon, &c. 



The order of the colours in thefe corona is as follows : 

 I. Next to the body of the luminary a circle of gravim 

 black, grayifli blue, or faint diluted white, altering as it 

 recedes into — 2. A ftrong lucid white of considerable breadth. 

 3. Slender rings of yellow and red. 4. A fu.cceffion of rin^s 

 of violet, blue, green, yellow, red. 5. Green, diluted yel- 

 low, red ; diluted green, diluted red. This is the common 

 order; but it is occafionally varied by flight irregularities. 



Newton has made the only plaufible attempt to account 

 for thefe appearances. He Suppofes them to be produced 

 by the rays of light falling upon globules of water, hail, or 

 ice. He aSTumes that globules of ice or hail may, or ac- 

 tually do. exift in Situations in the atmofphere in which thev 

 can receive the rays, fo as to produce that analyfis of the 

 white light, and that distribution of the coloured rays which 

 thefe coronas exhibit. He conjectures that thefe coronse in 

 the atmofphere, Surrounding the fun or moon, may be 

 produced in the fame manner in which a miniature ap- 

 pearance Somewhat Similar is occasioned by the reflexion of 

 light from a lens quick Silvered on the back-fide. 



But we do not know particles of hail or ice always to exift 

 in the atmoSphere when theSe many-coloured irides or co- 

 rona; appear. The ordinary refraction of direct light, in its 

 paflage through globules of liquid water, could not produce 

 thefe appearances. Newton's theory, though all that he 

 postulates were granted, would not explain that particular 

 distribution of thefe colours in the corona;, by which intervals 

 or vacant fpaces are left between the different portions of the 

 red and of the other colours. The iris, from a globule of 

 cryftal having quicksilver on its further Side, is, according to 

 Newton, produced, not by the reflexion of the main beam of 



light, 



